The invention relates to a hybrid vehicle drive system having a primary power source, such as a conventional internal combustion engine, and another power source, such as an accumulator containing high pressure fluid. More particularly, the invention pertains to vehicle hill holding using a hydraulic drive system.
Hydraulic Power Assist (HPA) is a type of hydraulic hybrid vehicle, in which energy from regenerative braking or from an engine is stored in a hydro-pneumatic accumulator, and the conversion between mechanical power and hydraulic power is achieved through high pressure pump/motor having a variable volumetric displacement. In an HPA system, using stored energy from regenerative braking to help accelerate the vehicle reduces the burden on the engine and reduces fuel use.
Because of the high power density available with such hydraulic systems, it is possible to recover efficiently a significant portion of braking energy with an HPA system comprised of a single pump/motor and storage accumulators. With a 7000 lb. vehicle and a pump/motor whose maximum displacement is 150 cc., an HPA system can recover 72 percent of the available braking energy in the Environmental Protection Agency (EPA) city cycle. The pump/motor operates for long periods at higher displacements and with a relatively high cycle average efficiency of 88 percent. With a return of 56 percent of the braking energy to the drive wheels (72 percent recovered in braking, and 88 percent transfer efficiency in both pumping and motoring), it is possible to recover 56 percent of the vehicle kinetic energy (or 75 percent of the velocity) while accelerating, neglecting road load friction. In the EPA city cycle it was possible to fill the hydraulic system when braking from 30 mph and then moderately accelerate again to about 22 mph using only stored energy from the HPA system.
U.S. Pat. No. 5,505,527 describes a vehicle powertrain having regenerative braking that includes wheels and a brake pedal which, upon engagement, is activated first into a first zone of operation and then into a second zone of operation. A braking detector detects either a released state or an engaged state for the brake pedal and, if in the engaged state, detects if the pedal is in the first or the second zone of operation. Friction brakes brake a pair of the wheels, responsive to detection of the brake pedal within the second zone of operation. The hydraulic portion of the drivetrain includes an accumulator for storing hydraulic fluid under pressure, and a reservoir for storing the hydraulic fluid at a lower pressure. A pump/motor, located in the high pressure line, operates as a motor to drive the drive wheels in a drive mode and as a pump driven by the drive wheels in a braking mode. A prime mover, having its inlet connected to the reservoir through a low pressure line and its outlet connected to the accumulator through a high pressure line, hydraulically drives the pump/motor in its motor mode. A controller switches the pump/motor into the braking mode in responsive to detection of an engaged state for the brake pedal and into the drive mode in responsive to detection of the released state of the brake pedal. A switch valve connects the high pressure line to the accumulator in the braking mode and to the reservoir in the drive mode.
In a hybrid hydraulic vehicle, vehicle braking may be accomplished by a combination of regenerative braking of the driven wheels and friction braking of all the wheels. Regenerative braking occurs when the vehicle is slowed or braked by wheels that drive a variable displacement pump/motor, which pumps hydraulic fluid into an accumulator. When the vehicle speed is too slow for regenerative braking to be effective, the friction brakes may have insufficient braking capacity at the expected brake pedal position. Then the driver may have to exert an uncomfortable level of force on the brake pedal to produce the level of friction braking desired. This condition is most evident when the vehicle is stopped and an automatic transmission in a forward drive or reverse drive range. In this case, displacement of the hydraulic pump/motor is increased to produce slightly more torque at the wheels than the torque transmitted there by an idling engine driving a torque converter, the automatic transmission, driveshaft and axle shafts. In this way, with minimal brake pedal effort, the vehicle is prevented from creeping or rolling inadvertently on the road surface. If, however, the vehicle is on a hill or for some other reason begins to roll despite the pump/motor operating at maximum displacement, the pump/motor displacement is returned to zero, and the driver must depress the brake pedal further to more fully apply the friction brakes and reduce regenerative braking. If the pressure in the high pressure accumulator, which stores energy recovered from vehicle kinetic energy while braking, is low, then a small amount of pumping under these conditions may be allowed.